Sealed secondary batteries, including aqueous electrolyte secondary batteries typified by high-capacity alkaline storage batteries and nonaqueous electrolyte secondary batteries typified by lithium-ion secondary batteries, are widely used as power sources for driving mobile equipment or other devices.
These sealed secondary batteries have a sealed structure in which an electrode group formed by stacking or winding a positive electrode plate and a negative electrode plate with a separator interposed therebetween is housed in a battery case together with an electrolyte, and in which an opening of the battery case is sealed with a sealing plate with a gasket sandwiched therebetween. In this structure, a lead extending from one of the electrode plates (e.g., the positive electrode plate) in the electrode group is joined to the sealing plate also serving as an external terminal at one side, whereas a lead extending from the other electrode plate (e.g., the negative electrode plate) in the electrode group is joined to the battery case also serving as an external terminal at the other side. To join the lead to the sealing plate or to the battery case, resistance welding is widely employed.
The opening of the battery case is sealed by resistance-welding the lead extending from the electrode group to the sealing plate, with the electrode group being housed in the battery case, and then bending the lead to seal the opening of the battery case with the sealing plate.
In this process, while the lead is resistance-welded to the sealing plate, substances (mainly metal particles removed from the melted lead) can be sputtered. If these sputtered substances enter the electrode group in the battery case, the separator might be damaged, resulting in an internal short. In another case where sputtered substances adhere to the gasket joined to the periphery of the sealing plate, when the opening of the battery case is sealed with the sealing plate by crimping with a gasket sandwiched therebetween, a narrowed portion of the gasket might be sheared by the adhering substances. Consequently, the battery case and the sealing plate come into contact with each other while sandwiching the adhering substances therebetween, resulting in a short circuit.
To prevent such a short circuit caused by, for example, contamination by sputtered substances, the opening of a battery case may be covered with a thin plate or the like so as to prevent sputtered substances from entering the battery case during resistance welding of the lead to the sealing plate. However, the opening cannot be completely sealed, and thus, such covering is insufficient for preventing contamination by sputtered substances.
On the other hand, joining by ultrasonic welding, instead of resistance welding, does not cause melting as caused by the resistance welding, and thus contamination by sputtered substances can be avoided in principle. However, joining by ultrasonic welding exhibits a lower joint strength than that obtained by the resistance welding. In addition, if the sealing plate has a safety mechanism for explosion protection, ultrasonic vibration might affect the function of the safety mechanism. Further, joining by ultrasonic welding is not preferable in reliability because an active material might be peeled off from the electrode plate.
Since positive electrode plates of lithium secondary batteries generally use aluminium, the leads extending from the positive electrode plates also use aluminium. In addition, to reduce the weight of batteries, the battery cases and the sealing plates have begun to use aluminium. In this case, a joint between the lead and the sealing plate means a joint between aluminium components. In general, an aluminium alloy has a higher electric conductivity and a higher thermal conductivity than those of steel. Accordingly, a large current needs to flow for a short period in resistance welding, resulting in that a welding rod badly wears, and it is difficult to maintain a stable joint for a long period. To prevent this problem, laser welding is employed for welding between the lead and the sealing plate (see, for example, PATENT DOCUMENTS 1 and 2).
This laser welding can considerably narrow a laser beam, thereby obtaining a small welding area. Accordingly, the amount of sputtered substances can be greatly reduced.